WO2015159268A1

WO2015159268A1 - Ferrochrome alloy production

Info

Publication number: WO2015159268A1
Application number: PCT/IB2015/052818
Authority: WO
Inventors: David Mitchell PHEIFFER; Kevin COOKSON
Original assignee: Sublime Technologies Pty Ltd
Current assignee: Sublime Technologies Pty Ltd
Priority date: 2014-04-17
Filing date: 2015-04-17
Publication date: 2015-10-22
Anticipated expiration: 2016-10-17

Abstract

This invention relates to a method of forming sintered and/or pre-reduced chromite agglomerates (typically briquettes or pellets) using and/or containing a SiC Composite (a form of silicon carbide containing material) as a reductant for use in the production of ferrochrome alloy. The method includes the steps of: preparing a mixture of particulate chromite, one or more binders and a reductant, in the form of SiC Composite alone, or preferably, SiC Composite in combination with another reductant such as coke, anthracite and/or coal or a mixture thereof; Forming the mixture into agglomerates, typically briquettes or pellets; and heating the agglomerates to a temperature from 800°C to below 1600°C to form sintered and/or pre-reduced agglomerates.

Description

FERROCHROME ALLOY PRODUCTION

BACKGROUND OF THE INVENTION

THIS invention relates to ferroalloy production and more particularly ferrochrome alloy production.

Ferrochrome alloy is produced by smelting chromium ore/agglomerates and/or concentrate in the presence of one of more fluxing agents and a carbon reductant in a DC arc, AC brush arc or AC submerged arc furnace. Most of the chromium and iron, together with some silicon and carbon, report in the metal phase known as ferrochrome alloy. The reductant which is used in smelting chromium ore/agglomerates or concentrate is carbon in the form of coke, coal, anthracite or the like.

International Patent Publication No. WO 01/79572 describes a process for producing ferrochrome alloy using a reductant which is carbide alone or carbide in combination with another reductant. It is provided that the preferred carbide is silicon carbide (SiC). SUMMARY OF THE INVENTION

This invention relates to a method of forming sintered and/or pre-reduced chromite agglomerates (typically briquettes or pellets) using and/or containing a SiC Composite (a form of silicon carbide containing material) as a reductant for use in the production of ferrochrome alloy. The method includes the steps of:

1) Preparing a mixture of particulate chromite, one or more binders and a reductant, in the form of SiC Composite alone, or preferably, SiC Composite in combination with another reductant such as coke, anthracite and/or coal or a mixture thereof.

2) Forming the mixture into agglomerates, typically briquettes or pellets (with a size of 5mm to 20mm); and

3) Heating the agglomerates to a temperature from 800°C to below 1600°C to form sintered and/or pre-reduced agglomerates.

In the preferred embodiment of the invention the SiC Composite is used in combination with coke, coal or anthracite or a combination thereof.

SiC Composite is a reductant containing silicon carbide (SiC) of between 35% and 98% SiC, preferably 75% to 90%, most preferably between 83% and 90%. SiC Composite contains fixed carbon of between 1 % and 15%, preferably between 2% and 10%, most preferably between 3% and 8%. SiC Composite also contains ash forming components (CaO, MgO, Si02, AI203, FeO) of 3% to 12%. SiC Composite has a loss on ignition of 1% to 2%.

The particulate chromite, and a reductant in the form of SiC Composite alone, or SiC Composite in combination with another reductant/s preferably has a particle size with 100% passing 125 μιη, more preferably 80% passing 75pm.

Typically, the chromite ore/concentrate should have a particle size with 90% passing 10mm, preferably 90% passing 2mm, most preferably 90% passing 1 mm; the , the SiC Composite has a particle size of less than 10mm, preferably less than 6mm, most preferably less than 1 mm, and these materials are ground together, or separately, with coke/coal/anthracite fines to the particle size mentioned above. The binder for pelletising is usually bentonite powder while for other agglomerates it can be cement, molasses, sodium silicate, lime or a combination thereof.

3% to 15%, preferably 4 to less than 10%, more preferably 4% to 7% by mass SiC Composite is added to the agglomerate mixture.

0% to 15%, preferably 0.2% to 12% by mass coke/coal/anthracite is added to the agglomerate mixture.

In the case of pelletising 0.5% to 2.5%, typically 0.8% to 2% by mass bentonite is added to the agglomerate mixture. In the case of briquetting the amount of binder is a function of the binder combination chosen.

The sintering step preferably takes place at a temperature between 800°C to 1350°C, preferably from 1 100°C to 1250°C, most preferably 1140°C to 1200°C.

The gas atmosphere in the sintering step is generally oxidizing comprising a mixture of C02, N2 and 02.

The sintering step is preferably followed by a pre-reduction step which takes place at between 1200°C and 1600°C, preferably from 1250°C to 1450°C.

The gas atmosphere in the pre-reduction step is oxidizing, neutral or, preferably, reducing, typically comprising a mixture of CO, N2 and CO_z.The reducing atmosphere contains typically more than 10% CO, preferably more than 15% CO.

A second aspect of this invention relates to sintered chromite agglomerates (typically pellets) produced by a method described above, for use in the production of ferrochrome alloy, containing SiC, typically 2% to 15% by mass SiC, preferably 4% to 7% by mass SiC.

A third aspect of this invention relates to a method of producing ferrochrome (FeCr) in a furnace by smelting agglomerates described above, wherein:

the agglomerates form at least 65% of the chromite feed and the balance lumpy and/or small lumpy ores; the SiC containing pellets form up to 100% of the chromite feed;

the requirement of carbon reductants is reduced by more than 15%;

the electrical energy requirement per FeCr unit is reduced by at least 11 %;

the Soderburg electrode consumption requirement is reduced by at least 40%;

the production rate per furnace is increased by at least 1 1%;

the use of coke as one of the reductants in the SAF is reduced to a minimum of 20% of the carbon based reductant blend;

the addition on Quartz is reduced by up to 70%.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph of thermogravimetric (TG) curves for chromite reduced by different proportions of coke and SiC Composite;

Figure 2 is a graph of TG curves for chromite reduced by different proportions of coke and

SiC Composite;

Figure 3 is a graph of TG curves for chromite reduced by the same proportion of coke and

SiC Composite for different durations;

Figure 4 is a graph of realized mass losses (from TG tests) versus expected mass losses

(from stoichiometry);

Figure 5 is a graph of TG curves for chromite reduced by the same proportions of SiC in argon and in CO;

Figure 6 is a graph of SiC Composite as a fraction of SiC Composite and coke versus the onset or reduction;

Figure 7 is a graph of TG curves for chromite pellets containing only coke binder;

Figure 8 is a graph of TG curves for chromite pellets containing only coke binder and SiC

Composite; Figure 9 is a graph of TG curves for chromite pellets containing both SiC Composite and coke as reductant;

Figure 10 is a graph of TG curves for chromite reduction under different conditions at

1350°C;

Figure 11 is a graph of TG curves for chromite reduction under different conditions at

1500X; and

Figures 12-14 are electron microscope photographs of pre-reduced chromite pellets of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

This invention relates to an improved process for producing a ferroalloy, in particular ferrochrome alloy.

SiC Composite is a reductant containing silicon carbide (SiC) of between 35% and 98% SiC, preferably 75% to 90%, most preferably between 83% and 90%. SiC Composite contains fixed carbon of between 1 % and 15%, preferably between 2% and 10%, most preferably between 3% and 8%. SiC Composite also contains ash forming components (CaO, MgO, Si02, AI203, FeO) of 3% to 12%. SiC Composite has a loss on ignition of 1% to 2%. In accordance with the present invention chromite, a binder or combination of binders and a reductant in the form of:

- SiC Composite alone; or

- More preferably, SiC Composite in combination with another reductant such as coke, coal or anthracite or a combination thereof;

Are combined, milled to typically 80% passing 75 microns and formed into agglomerate, preferably a pellet, which is first sintered under oxidising conditions, preferably but not essentially, followed by a pre-reduction process under reducing conditions to form sintered and pre-reduced pellets for the production of ferrochrome. The pre-reduction process may take place inside a Submerged Arc Furnace (SAF), Brush Arc Furnace (BAF) or in a separate specialized production plant such as a Rotary Kiln. The formation of sintered pellets for the production of ferrochrome alloys is well-known. The sintered pellet structure is durable and withstands mechanical treatment and also transforms the grain structure with iron in the chromite, oxidising significant amounts from Fe^2* to Fe³⁺, which facilitates the subsequent pre-reduction and reduction processes and when the pellets are smelted in a furnace.

In accordance with the present invention it has been found that the SiC Composite does not significantly degrade/oxidize during the sintering process. Furthermore the current invention has found that the SiC Composite actually achieves some pre-reduction of chrome and iron during the sintering process. It has also been found that the size of the SiC Composite particles have an effect on the subsequent pre-reduction of the pellets, and that the pre-reduction atmosphere is also important.

Chromite typically contains MgO (8-22%), Al₂0₃ (7-16%), Si0₂ (2-10%), CaO (0.2-1%), Cr₂0₃ (34- 65%), and FeO (5-35). All percentages by mass of the chromite.

The coke may have a carbon (C) content of 78-85% by mass, typically about 82% by mass. Anthracite has a C content of 70-85% by mass, typically about 80% by mass. Coal has a C content of 50-70% by mass, typically about 55% to 65% by mass.

In a preferred embodiment of the invention, the reductant is SiC Composite in combination with coke, coal and/or anthracite or a combination thereof. Chromite is ground in a grinding ball mill together, or separately, with coke/coal/anthracite fines and SiC Composite fines to a predetermined size. The size may be 100% passing 125 μητι, preferably 80% passing 75pm. The chromite may be ground separately then mixed with particulate SiC Composite. In all cases a binder, typically in the form of bentonite powder, is added.

The amount of SiC Composite and coke added to the chromite may be calculated stiochiometrically based on the chemical analysis of the chromite in the smelting furnace feed utilising the following chemical reactions, as corrected for the purity of each of the components and the degree of reduction required:

FeCr₂0₄ (ideal stoichiometry of chromite) Overail: FeCr₂0₄ + SiC → Fe + 2Cr + Si0₂ + CO Overall: FeCr₂0₃ + 3C → Fe + 2Cr + 3CO

Typically 3% to 15%, preferably 4% to 7% by mass SiC Composite is added and 0% to 15%, preferably 0.2% to 12% coke/coal/anthracite is added to the agglomerate. The ratio of reductant chosen is dependent on the amount of pre-reduction to be achieved in the agglomerate prior to final reduction and smelting in the DC/SAF/BAF.

Usually from 0.5% to 2.5% by mass bentonite, typically 0.8% to 2% by mass bentonite, is added.

The SiC Composite is preferably crushed to a size less than 10mm, preferably less than 6mm, most preferably less than 1 mm prior to milling to typically 80% minus 75 microns.

The milled chromite, coke/coal/anthracite, SiC Composite, and bentonite are mixed and passed to a pelletising drum/disc where green pellets, having a typical size of 8mm to 15mm, are formed. The green strength of the pellets is not adversely affected by the addition of SiC Composite. Furthermore, owing to the carbon in the SiC Composite, it may not be necessary to add additional carbon to the pellets, again depending on the level of pre-reduction to be achieved. The green pellets are then passed to a sintering furnace which is operated at a temperature from 800°C to below 1400°C, preferably above 1 100°C to 1200°C, in an oxidizing atmosphere. There is no significant adverse effect on the pellet strength when adding SiC Composite to the pellet. Some pre-reduction of Cr/Fe may occur during this sintering process.

The atmosphere in the sintering step is oxidising while that for the pre-reduction step is, preferably, reducing in a mixture of CO and C0₂, preferably the reducing atmosphere contains more than 10% CO, preferably more than 15% CO.

Smelting tests conducted on the pellets, prepared according to the invention, at temperatures above from 1500°C to 1725°C, have found that the pre-reduction temperature and particle size of the SiC Composite have a significant effect on the degree pre-reduction. The pre-reduction atmosphere is also important. The invention will be described in more detail with reference to the following non-limiting examples.

Example 1 - Laboratory Scale Thermo Gravimetric Tests to show the effect of SiC Composite during sintering and pre-reduction.

Thermo gravimetric (TGA) tests are carried out in a thermo balance. The apparatus has three parts, which measure and control three variables, namely mass, temperature and atmosphere. The sample is contained in an alumina crucible, which is supported on an alumina rod connected to the load-cell pin of a top-pan balance. The sample holder and supporting rod stand in a work tube, a ceramic tube through which passes a controlled atmosphere. The work tube is supported in a furnace, which supplies the energy (temperature) needed for the reactions. Tests were conducted in a neutral argon atmosphere metered through a variable flow meter. Temperature was increased at 5°C/min. It was held at 1600°C for a time period from 15 minutes to 3 hours before the furnace was allowed to cool at 5°C/min. A thermocouple placed a centimeter above the sample holder measured the temperature of the sample. Temperature and sample mass were logged at 15 second intervals. From these measurements a TGA curve is inscribed.

A number of tests were conducted, Table 1 summarizes their conditions. All but one test was conducted under a flow of argon. The proportions of chromite, coke and SiC Composite varied. Temperature was raised to 1600°C and held there for 15 minutes, 120 minutes or 180 minutes before it was ramped down again. In each test every effort was made to bring particles of chromite, coke and SiC Composite into close contact with each other. Nevertheless, because of differences in density and particle size, the dso of the reactants differed; it was 185 pm for chromite, 1.1 mm for SiC Composite, and 2.8 mm for coke,— some segregation of reactants was seen to occur as the sample holder was charged with a sample for testing.

Pellets of chromite prepared according to three recipes were tested in the thermo balance. The three recipes were:

• Chromite + 2% binder

• Chromite + 2% binder + 16.6% SiC Composite

• Chromite + 2% binder + 16.6% SiC Composite + 4.1 % coke

The temperature was ramped up at 5°C/min to 250°C, held there for 15 minutes, and then allowed to cool at 5°C/min.

The TGA curves indicate the following points about the tests and the role of the reductants in the reaction:

1. The reaction (i.e., reduction of chromite) with a mix of SiC Composite and coke starts at a lower temperature than that of either coke or SiC Composite alone (see Figures 1 and 2). With reductant additions near the stoichiometric requirement (tests 1-7), the difference is one of 1200°C versus 1420°C (Figure 1). With reductant additions significantly above the stoichiometric requirement (tests 15-17), the difference is one of 1240°C versus 350°C (Figure 2).

2. After 15 minutes at 1600°C, chromite with only SiC Composite as reductant is more reduced, 25.5% mass loss, than chromite with only coke as reductant, 14.8% mass loss (Figure 1). There is more reaction in the first 15 minutes (10.8% hour) at 1600°C than the 3.3%/hour after 90 minutes (see Figure 3, which shows TGA curves measured under similar conditions but for different durations). . Under a mixture of SiC Composite and coke (in excess of stoichiometric requirements) reduction is complete in 3 hours (see Figure 4). With coke alone the reaction is incomplete after 3 hours. . In the first 15 minutes at 1600°C there is little difference in the rate of reduction of chromite by SiC Composite in an atmosphere of Argon than in one of CO. See Figure 5.

5. The fraction of SiC Composite in the reductant added to reduce chromite does have an effect on the lowest temperature at which reduction becomes significant: from a high of ~1400°C for pure coke it drops to ~1250°C for SiC Composite of 15-30% of the reductant before rising again for higher levels of SiC Composite (see Figure 6).

6. For pelletising, there is no apparent difference in behaviour for the sample containing only coke (as a binder, Figure 7), and that containing coke and SiC Composite (Figure 8), when heated for 15 minutes in an oxidising atmosphere. However, where SiC Composite and coke are added as reductant, significant reduction occurs (Figure 9).

In the case of the first two tests the reduction / metallisation is reversed during cooling, in the sample where both SiC Composite and coke reductant was present, not all the reduction was reversed during cooling.

In conclusion the following important points emerged during the investigation;

• SiC Composite can substitute for carbon reductants during the reduction of ferrochromium.

^• The overall mass loss using SiC Composite is less owing to the formation of silica as one of the products of reduction.

^• The volume of carbon monoxide gas produced using the SiC Composite is lower than with carbon based reductants owing to the silicon based reduction reactions taking place.

^• Where SiC Composite and coke are used together as reductant there is a marked increase in the reaction rate, and more importantly, reduction starts at up to 200°C lower than where either of the reductants is used on its own. The decrease in reaction temperature is evident at all SiC Composite levels but is greatest where SiC Composite constitutes between 10 and 40% of the total reductant.

• The increase in reaction rate observed where SiC Composite and coke are used in combination is a solid state effect, a similar increase is not observed if SiC Composite is used in a CO atmosphere.

^■ SiC Composite can be incorporated into pellets without inducing any significant change in the quality of the sintered pellets. Example 2 - Further Laboratory Scale Tests

This Example provides the results of a further series of laboratory scale TGA tests, one in which the reduction of three recipes of chromite (nominally FeCr₂0₄) and reductant (coke and SiC Composite), rolled into pellets, was assessed at two temperatures (1350°C and 1500 °C) and three external conditions:

In contact with CO

In contact with CO and coke

In contact with CO, Coke and SiC Composite

The tests were designed to simulate the typical sinter steel belt process. Samples and Procedures

The raw materials (chromite, SiC Composite and coke) were combined and rolled into pellets (to simulate the typical chromite pelletising and sintering steel belt process) according to three recipes:

Recipe 1 : chromite + binder (graphite at 2% of chromite)

Recipe 2: chromite + binder (2%) + SiC Composite (30% of stoichiometric requirement)

Recipe 3: chromite + binder (2%) + SiC Composite (30% of stoichiometric requirement) + coke (20% of stoichiometric requirement)

The masses of constituents in each recipe amounted to - Recipe 1 : 100 g chromite + 2 g binder

Recipe 2: 100 g chromite + 2 g binder + 6.6 g SiC Composite

Recipe 3: 100 g chromite + 2 g binder + 6.6 g SiC Composite + 4.2 g coke

The pellets ranged in size from about 10 to 15 mm in diameter. The pellets had sufficient green strength for handling. 80% of constituent pellet particles passed 75 pm.

The tests were conducted in a thermo balance. The sample holder, a crucible of sintered alumina, held several pellets. The tests explored reduction at two temperatures, 1350°C and 1500 °C. The pellets were heated in an atmosphere of CO at ~5°C/min, and held at temperature for four hours before power to the furnace was cut and the sample left to cool with the furnace. In six of the tests the space between pellets in the crucible was filled with coke. In two of the tests the space was filled with a mixture of coke and SiC Composite. The tests were run in sequence:

The temperature and the mass of the sample were continually logged during a test. The information is expressed in a TGA curve. The basis for each curve is the initial mass of the pellets alone. The implication is that the final mass loss in tests where SiC Composite and coke were added to the crucible may be more than it would have been had these reductants not been added (some carbon from SiC Composite and coke may be consumed in reduction).

Results

The TGA curves (see Figures 10 and 11) indicate the following facts about the tests and, by inference, about the role of reductants (SiC Composite, coke and CO) in the reduction reaction:

1. In all of the tests reduction remains incomplete even after 4 hours at temperature (see Figures 10 and 11)

2. The final mass lost by recipe 3 in an atmosphere of CO (tests 1 a and 1b) is the same at 1350°C and 1500 °C, yet the degrees of Fe and Cr metallization are much higher at the higher temperature.

3. The degrees of Fe (90%) and Cr (25%) metallization in test 1b are similar to those test 2a. 4. Fe and Cr metallisation in test 3a are significantly higher than for test 5a. A similar significant trend is recorded for tests 2b, 3b and 5b.

5. Recipes 2 and 3 show similar mass losses during reduction at both 1350°C and 1500°C. Recipe 1 under these conditions undergoes a similar mass loss as the other two recipes at 1350°C, but loses comparatively less mass at 1500°C.

6. Chromite is reduced by coke surrounding the pellets, but the extent of reduction is lowered by replacing some of the coke with SiC Composite (compare tests 4 and 5).

Conclusions

The following conclusions emerged from this investigation:

There is a considerable increase in the degree of pre-reduction in the pellets where SiC Composite and carbon based reductants are present, in particular at the lower temperature of 1350 °C.

Example 3 - Pilot Scale Chromite Pellets

The compositions of the chromite, coke and SiC Composite used in these tests is in Table 2.

Table 2 - Bulk chemical composition of chromite, coke and SiC Composite

Three different recipes for the preparation of the chromite pellets were used. These would be used to conduct a total of five tests at various conditions. The recipes are summarised in Table 3. Table 3 - Pellet Recipes

Smelting of the pellets

Five tests were conducted to assess the effectiveness of SiC Composite as a reductant for the production of ferrochromium alloy using the chromite pellets of the three recipes. The pellets were treated as follows:

Table 4: Sintering, Pre-reduction and Smelting Tests

Pre-Reduction

Test Recipe Coke Temp Time Reduction

Atm

Stoic Deg C hrs % Cr % Fe

1 2 None

2 1 Yes 1350 10%CO, C02 2.5 95.0 99.0

3 1 Yes 1500 10%CO, C02 2.5 95.0 99.0

4 1 Yes 1350 10°/oCO, CO 2 24 50.0 99.0

5 3 Yes 1350 10%CO, CO 2 24 50.0 99.0

Test Observations:

Test 1 : The pellets remained intact after smelting at the furnace set point of 1675°C resulting in the production of a relatively small amount of ferrochromium alloy without slag.

Test 2: The residual pellets could be seen resulting in the formation of a small mass of the alloy with no slag. It is also very important to note that the preservation of a 90% C0₂ and 10% CO atmosphere was impracticable in Tests 2 and 3 due to formation of additional CO from the calcination of coke.

Test 3: This test was conducted at 1725°C in the belief that a further 50°C would help to improve the melting of the chromite pellets. The pellets were completely melted. The crucible had leaked yielding only the alloy without slag.

Test 4: It was very difficult to maintain the heat of the sample after sintering and pre-reduction as required as there was a need to change the furnace and the crucible after each of these steps. To avoid thermal shocking on the crucible due to vigorous heating, the ramping rate was lowered. This helped to eliminate the temperature discrepancy between the furnace set point and the sample. Nevertheless the crucible still leaked. The leak on the crucible could also be attributed to the swelling of coke and SiC on heating.

Test 5: The crucible in this test leaked. Alloy and slag were produced, the slag contained small dots of the alloy. RESULTS AND DISCUSSIONS

Loss on ignition tests

The loss on ignition tests were undertaken on: the dried pellets of the three recipes, sintered pellets for Tests 1 to 3 and the pre-reduced pellets in Tests 2 and 3.

Table 5 - Drying, sintering and pre-reduction mass losses

Compression strength test

The compression strength of Recipe 1 and Recipe 2 pellets was evaluated after drying and sintering. The results are summarised in Table 6.

Table 6 - Compression strengths results for the chromite pellets

Chemical compositions of the pellets of the three recipes

The chromite peiiets of three recipes prepared for conducting various studies were analysed to determine their chemical composition in the agglomerated form. Table 7 gives a summary of the results of the analysis of the pellets.

Table 7: Chemical composition of the "as prepared" peiiets

Sintering tests:

Table 8 - Chemical composition of the sintered pellets

Pre-reduction tests:

One of the main aims of the tests was to determine the degree of pre-reduction that had occurred in the pellets under conditions simulating a pelletising sinter belt. The chemical compositions of the pre-reduced pellets for Test 2 to 5 are summarised in Table 9 (Test 1 was a control test with no reductant added so has no pre reduction results). Included in Table 9 is the Fe and Cr metallic content as determined by standard wet chemical techniques. There is clearly significant variation between the reported metallisation and what might be expected from the original recipes (the degree of Fe reduction as a per cent of the starting Fe is given in the last column of Table 9). For example, in Recipes 1 and 3 (i.e. tests 2 to 5) sufficient SiC Composite was added to lead to 25% reduction (bearing in mind that there are also 6% coke added as a binder / reductant - which is roughly equivalent to a further 25% reduction). ln all cases Fe reduction is thermodynamically favored over Cr reduction so it is fully expected that the Cr metal content would be low. In the case of Test 2 the degree of iron reduction is low at 15% conversion of Fe oxide to Fe. In contrast, for Test 3 where the pre-reduction temperature was increased from 1350°C to 1500°C, the degree of reduction increased to 63%. This is at the high end of what might be expected and suggests near 100% efficiency of the reductant (SiC Composite and coke), a fact that is supported by the low carbon content of the pre-reduced pellets.

Test 4 was differed from Test 2 in that the atmosphere was CO as opposed to a 10% CO / 90% C02 mixture. In this case the degree of iron reduction has improved to about 23%, showing that the atmosphere does have a material effect on the degree of iron reduction (it has increased by about 50%). There are two possible reasons for this. Firstly, the more reducing atmosphere may prevent the loss of coke due to oxidation, thereby resulting in a greater amount of reductant available for metallurgical reduction (it must be remembered that early tests showed that reduction is enhanced in the presence of both carbon and SiC Composite). The second possible reason is that CO plays a part in the reduction mechanism (which it is known to do) and the greater partial pressure of CO increases the decree of reduction. Both of these tests have similar residual carbon contents suggesting that carbon loss cannot be the only explanation. There is however implications for the efficacies of adding reductant to pellets that will be processed using conventional sinter belt technology.

Finally, Test 5 utilized SiC Composite of a much finer size and this had a significant effect on the degree of iron reduction. When compared to Test 4 with the coarser material, the amount of iron reduction in Test 5 is more than double at almost 48%. This also approaches the theoretical degree of reduction of the combined SiC Composite and Coke (including the 2.5% binder Coke). There is also a material increase in the degree of chromium reduction illustrating the greater efficiency of finer SiC Composite.

Table 9: Chemical composition of the pre-reduced pellets

Following the initial analysis of the pre-reduced pellets, the analytical results indicated that there was no metallisation. A number of the tests were repeated to confirm the results but it became clear that reduction was occurring but the analytical method was not able to detect it. Initially two types of confirmatory testing were undertaken, magnetic susceptibility measuring and a microscopic analysis. The results of the electron microscope based analysis are shown in Figures 15 to 20, which clearly indicate the presence of metal.

Following further x-ray diffraction analysis it was identified that some of the 'metal' had adopted a 'carbide' structure, in an effort to exclude experimental artifacts as the reason for this the tests were repeated with slow cooling and quenching of the pellets. Further analysis of these products showed no difference and thus experimental artifacts were excluded as an explanation, in the meantime the analytical problems were resolved and suitable analytical results were produced. The reason for the formation of carbides is however curious given that the carbon content of the 'metal' was low, well below that normally found in high carbon ferrochromium.

Example 4: INDUSTRIAL TRIAL

The findings of the laboratory and pilot scale test work described above where tested on a ferrochrome production facility on a full industrial scale. A 400 OOOtpa petletising and sinter plant was used to prepare SiC Composite containing sintered pellets and a 66MVA closed SAF used for smelting. The trial ran for 2 months.

The basis for the trial was to feed the SAF with 70% and 65% SiC Composite containing pellets as a percentage of the chromite ore feed with the balance of the chromite fed as lumpy and/or chip ore. The SiC Composite added to the pellets was targeted to achieve a SiC replacement of the SAF carbon based reductants by between 20 and 25%.

Pelletising: The wet milling plant was fed with a blend of chromite concentrates consisting of LG6, MG1 , MG2, MG4 and UG2 type ores depending on ore availability. Various spillage and screened undersize materials were also fed to the mill. An amount of SiC Composite was fed to the mill calculated based on the percentage of carbon to be replaced by SiC in the SAF and adjusted for the SiC Composite purity. The SiC Composite varied from 84% to 89% contained SiC. Depending on the free carbon content of the recycle materials the amount of new carbon containing materials fed to the mill was adjusted to maintain the desired carbon content in the green pellets. The feed materials were milled concurrently with each other to a nominal size of 80% < 75 microns. The slurry produced was filtered by ceramic filters to produce a filter cake with less than 10% moisture content. The presence of SiC was noted to enhance the filter performance with a drier filter cake and less cleaning time required. The filter cake was then blended with bentonite as a binder and, optionally, with water and carbon duff, and formed into green pellets in a pelletising drum. The green pellets were 8 to 16mm in size. The green pellets were then fed to a sintering furnace to harden the pellets. Energy for sintering is supplied by the green pellet contained free carbon, some SiC reaction and SAF CO gas and/or LPG. Less than 2.5% of the SIC was found to react in the process. The sintering furnace has a number of zones: drying, pre-heating, sintering, balancing and cooling zones 1 to 3. The sintering zone temperature was controlled between 1 100 deg C and 1350 deg C with best results achieved below 1250 deg C. It was determined that the sintered pellet strength could be adjusted by changing the bentonite addition to the green pellets. Once optimized the sintered pellet quality was acceptable with strengths above 220kg and tumble test fines less than 3.5%.

The operating results and parameters are summarised in tables 10 and 11 below:

Table 10: Plant feed

Table 11 : Pellet quality

The sintered pellets were screened and stockpiled ahead of the SAF. The typical chemical analysis was: Table 12: Pellet analysis

Furnace: The SIC containing pellets were fed to the SAF initially at 70% of the chromite ore feed. The balance being lumpy ore 10 to 180mm in size. The pellets contained sufficient SiC to replace 20 to 25% of the norma! carbon required in the SAF. Furthermore, during the trial it was possible to reduce the coke from 38% of the carbon based reductants to 20%, decreasing the overall cost of carbon based reductants by nearly 50%.

Approximately half way through the trial the pellet ratio was reduced to 65% of the chromite ore fed to the SAF. The balance was lumpy ore. The results of the two trial periods were compared to a reference sample which was the period just prior to the trial. The results are compared in Table 13.

Table 13: SAF Trial results

Overall an increase in 1 1 % in production volume was achieved together with a 5% reduction in production cost. The trial was deemed to be a technical and financial success.

Particular note needs to be made of the high ore consumption while using SiC containing pellets. This was owing to the poor lumpy ore quality (MG4 used) and higher use of UG2 in the pellets. Preferably would be to operate the SAF on 100% SiC containing pellets.

An additional positive effect of operating with SiC containing pellets is the substantial reduction in the soderburg electrode consumption, by as much as 47%.

Claims

CLAIMS 1.. A method of forming sintered and/or pre-reduced chromite agglomerates for use in the production of ferrochrome alloy, the method including the steps of: a. preparing a mixture of particulate chromite, optionally a binder, and a reductant in the form of SiC Composite alone, or SiC Composite in combination with another reductant/s;

b. forming the mixture into agglomerates; and

c. heating the agglomerates to a temperature from 800°C to below 1600°C to form sintered and/or pre-reduced agglomerates.

2.. The method as claimed in claim 1 , wherein the agglomerates are pellets with a size of 5mm to 20mm.

3.. The method as claimed in claim 1 or 2, wherein the reductant is a SiC Composite in combination with another reductant/s.

4.. The method as claimed in any one of the preceding claims, wherein the other reductant is coke, coal and/or anthracite or any combination thereof.

5.. The method as claimed in any one of the preceding claims, wherein the SiC Composite contains, by mass, 35% to 98% SiC, 1% to 15% fixed carbon, and 3% to 12% ash forming components including CaO, MgO, Si02, AI203, and FeO.

6.. The method as claimed in claim 5, wherein the SiC Composite contains, by mass, 75% to 90% SiC, and 1 % to 10% fixed carbon.

7. The method as claimed in claim 6, wherein the SiC Composite contains, by mass, 83% to 90% SiC, and 3% to 8% fixed carbon.

8. The method as claimed in any one of the preceding claims, wherein the particulate chromite, and a reductant in the form of SiC Composite alone, or SiC Composite in combination with another reductant/s has a particle size with 100% passing 125 pm. The method as claimed in claim 8, wherein the particulate chromite, and a reductant in the form of SiC Composite alone, or SiC Composite in combination with another reductant/s has a particle size with 80% passing 75pm. The method as claimed in any one of the preceding claims, wherein the binder is bentonite powder. The method as claimed in any one of the proceeding claims wherein the binder is lime, cement, molasses, sodium silicate, bentonite or any combination thereof. The method as claimed in any one of the preceding claims, wherein 3% to 15% by mass SiC Composite is added to the mixture. The method as claimed in claim 12, wherein 3% to less than 10% by mass SiC Composite is added to the mixture. The method as claimed in claim 13, wherein 4% to 7% by mass SiC Composite is added to the mixture. The method as claimed in any one of the preceding claims, wherein 0% to 15% by mass coke/coal/anthracite is added to the mixture. The method as claimed in claim 15, wherein 0.2% to 12% by mass coke/coal/anthracite is added to the mixture. The method as claimed in any one of the preceding claims, wherein 0.5% to 2.5% by mass bentonite is added to the mixture. The method as claimed in claim 17, wherein 0.8 to 2% by mass bentonite is added to the mixture. The method as claimed in any one of the preceding claims, wherein the sintering step takes place at a temperature between 800°C to 1350°C. The method as claimed in claim 19, wherein the sintering step takes place at a temperature from 1120°C to 1250°C. The method as claimed in claim 20, wherein the sintering step takes place at a temperature of 1200°C. The method as claimed in any one of the preceding claims, wherein the atmosphere in the sintering step is oxidizing. The method as claimed in any one of the preceding claims, wherein sintering step is followed by a pre-reduction step takes place from 1250°C and 1600°C. The method as claimed in claim 24, wherein the pre-reduction step takes place from 1300°C to 1500°C. The method as claimed in any one of the preceding claims, wherein the atmosphere in the pre-reduction step is reducing. The method as claimed in claim 25, wherein the atmosphere in the pre-reduction step comprises a mixture of CO and C0₂. The method as claimed in claim 26, wherein the atmosphere in the pre-reduction step contains more than 10% CO. The method as claimed in claim 27, wherein the atmosphere in the pre-reduction step contains more than 15% CO. Agglomerates produced by a method as claimed in any one of the preceding claims, containing 3% to 15% SiC by weight. Agglomerates as claimed in claim 29, containing 4% to 7% SiC by weight A method of producing ferrochrome (FeCr) in a furnace by smelting agglomerates defined in claim 29 or 30. The FeCr method as claimed in claim 31 , wherein the agglomerates form at least 65% of the chromite feed and the balance lumpy and/or small lumpy ores. The FeCr method as claimed in claim 32, wherein the SiC containing pellets form up to 100% of the chromite feed. The FeCr of claim 30, wherein the requirement of carbon reductants is reduced by more than 15%. The FeCr method of claim 30, wherein the electrical energy requirement per FeCr unit is reduced by at least 1 1 %. The FeCr method of claim 30, wherein the Soderburg electrode consumption requirement is reduced by at least 40%. The FeCr method of claim 30, wherein the production rate per furnace is increased by at least 1 1 %. The FeCr method of claim 30, wherein the use of coke as one of the reductants in the SAF is reduced to a minimum of 20% of the carbon based reductant blend. The FeCr method of claim 30, wherein the addition on Quartz is reduced by up to 70%. A method as claimed in claim 1 , substantially as herein described with reference to any one of the examples.